The present invention relates to a mobile handling device with a hydraulic circuit, which hydraulic circuit comprises a lifting cylinder arranged in a lifting device intended for the handling of a variable load and an accumulator for recovering or recycling the lowering load energy, the hydraulic circuit also comprising a variable hydraulic machine with two ports, said hydraulic machine being able to give full system pressure by a driving device in two flow directions to said ports, wherein one of the ports is connected to said accumulator and the other port is connected to said lifting cylinder.
Excavators, trucks, container handlers etc. and a large number of other mobile handling machines which are intended to handle a variable load have one or more lifting cylinders for lifting the load for which the unit is designed. The great majority of mobile handling devices used today have no energy recovery facility whatever for the lowering load, meaning that the lowering load energy, most often in connection with passage via a control valve which determines the lifting and lowering motion, is converted to heat which then has to be cooled away. The heating of the hydraulic oil to undesirable temperatures is a long familiar problem for machinery manufacturers and end customers.
For several years, a plurality of inventors have been working on recovering the energy losses which arise in a lifting system without any ballast weight balancing away the weight of the arm system. For different reasons they have not managed to obtain a commercially useful solution, as all the time there exist weaknesses implying unacceptable results. Below, different reasons for said problems are mentioned.
A device with an auxiliary cylinder, which is more or less directly connected to one or more accumulators, creates difficulties, as the arm system to a large extent influences the hydraulic pressure in the lifting cylinder depending on the working radius used. The system must thus balance towards the lowest pressure which may exist at a short working radius, which is a problem.
In those solutions where attempts have been made with the lifting cylinder in a closed circuit together with a hydraulic machine, wherein oil is pumped to and from an accumulator, the problem is to compensate the leakage losses, which are unavoidable in all rotating hydraulic machines. When the hydraulic oil has run short in the accumulator, which happens simultaneously, the need of a power peak will immediately arise, which results in difficult problems, which to a great extent lessens the value of such a solution.
An object of the invention is to eliminate or at least minimize the above mentioned drawbacks, which object is achieved by a mobile handling device according to the characteristic part of patent claim 1.
The invention provides many advantages and i.a. a considerable reduction of the engine power. The invention has so far been tested in an excavator of the size 20 tons but is also applicable on practically all lifting devices.
At least the following important advantages are achieved with the invention:
1 At least the major part of the position energy, which is transformed when the arm system is lowered, is recovered.
2 Said position energy, which is transformed during the lowering step, is recovered to a large extent without being transformed to heat.
3 A comparatively low engine power can be installed by, during the lifting operation, utilizing the energy which have been stored during the lowering step, and preferably by utilizing in an optimal way the engine power to load the accumulator alternatively the accumulators, when the power is not utilized for any other purpose.